Older adults represent one of the fastest growing segments of the population, and by 2030, those aged 65 and older are expected to make up more than 20% of the US population. Parallel to this demographic shift, the US is experiencing an unprecedented increase in the number of people who are obese. This increase in obesity extends to those who are older as well, with over 37% of older adults expected to be obese by 2010. The cumulative effects of age- and obesity-related disease risk could create a tremendous problem for our already strained health care system. The risks of cardiometabolic diseases (e.g., diabetes, heart disease) and declines in functional status and quality of life for older adults will be extraordinarily high. One potential option to reduce these risks is to promote weight loss in older adults. However, this recommendation has limited evidence to support its benefit in this age group and may actually be harmful for older adults-weight loss can decrease lean muscle mass and bone mineral density, and higher body mass index has been associated with lower mortality in older adults. The key to understanding how to minimize risk with weight reducing diets in older adults may be in understanding how age-related changes in body composition, in addition to excess weight gain, affect cardiometabolic risk factors and functional status. Prior research suggests that visceral adipose tissue may be primarily responsible for driving obesity associated risk in older adults, leading to increased cardiometabolic disease risk and worsened functional status. If visceral fat is a key factor that increases risk in obese older adults, do weight reducing diets that decrease total fat mass as well as lean mass unnecessarily increase risk of an adverse outcome in the setting of ongoing age-related declines in skeletal muscle and bone mass? Ideally, older adults using a prescribed dietary intervention combined with exercise would be able to preferentially reduce visceral fat mass, while minimizing loss of lean muscle, to obtain maximum improvements in both cardiometabolic and functional health risk. We propose to conduct a randomized, controlled trial in 180 older adults with 1 year follow up to test the hypothesis that changes in diet composition alone or in combination with calorie restriction have a significant effect on visceral fat stores (primarily) in older adults and as a result have an impact on cardiometabolic and functional health risk factors and quality of life. Compared to an exercise only control group, we will assess the effects of weight maintenance and weight loss (10%) interventions on body composition (Specific Aim #1: total body fat, visceral fat, subcutaneous fat, muscle mass, intramyocellular lipid, bone density), cardiometabolic risk factors (Specific Aim #2: blood pressure, blood glucose, insulin, lipids, C-reactive protein, adiponectin, leptin, tumor necrosis factor-1, interleukin-6), functional status, and quality of life (Specific Aim #3: life space mobility, Short Physical Performance Battery, leg and hand grip strength, Short Form-36 Health Related Quality of Life, Impact of Weight on Quality of Life-Lite). PUBLIC HEALTH RELEVANCE: The significance of this project, including its relevance for public health, is the new evidence it will provide regarding whether changes in diet composition alone or in combination with calorie restriction benefits, without causing harm, obese older adults at high risk for cardiometabolic disease and functional impairment. Important results generated from this study can be used to inform both clinical decision-making and community-based public health initiatives designed to improve the quality of life of the increasing population of older adults who are living longer.